It is a known technique to remove flash from molded plastic and elastomeric articles and paint or coatings from various articles by contact with a chilling medium, generally at cryogenic temperature, to embrittle the flash or coating, and subjecting the chilled article to impact with a high velocity stream of shot or pellets.
In a typical operation the piece or pieces to be treated are introduced into a thermally insulated chamber maintained at required low temperature and the stream of blasting media is centrifugally impelled at high velocity against the surface of each workpiece by a rotating impeller or so-called throwing wheel. Systems of this general type are described, for example, in U.S. Pat. Nos. 2,996,846; 3,110,983; 3,824,739; 4,312,156; Canadian Pat. No. 1,112,048, and in copending application Ser. No. 445,778 filed Nov. 30, 1982. Systems are also known for blasting of workpieces, such as metal castings to be cleaned or deburred, with an abrasive medium, impelled against the surface of the workpiece by a throwing wheel, as illustrated for example, by U.S. Pat. No. 3,703,789.
In certain of these prior art systems the workpieces are moved through the treating chamber on an endless conveyor during treatment; other systems operate in a batch mode wherein a batch comprising one or a number of pieces to be treated are loaded into the treating chamber, subjected to impact with the blasting medium and then withdrawn to be replaced by a fresh batch. In the batch process, some form of support is provided for the workpieces, which may be stationary or may include means for rotating or otherwise moving the workpieces or the supporting fixture, to expose the several surfaces to the blasting media. Power means may be provided for opening and closing the door of the treating chamber during loading and unloading.
In the batch process for removal from workpieces of paint or coatings embrittled by chilling, a very short-cycle time needs to be employed. Thus, the door to the treating chamber must be opened and closed frequently while the system is at desired operating temperature. The workpieces to be treated may vary considerably in size and shape, so that it is desirable to provide a work-supporting fixture designed to accommodate the different types and sizes of workpieces being subjected to the blasting operation.
Since the coating removal cycle time is very short, generally in the order of only about 6 to 8 minutes, it is necessary for desired productivity to provide a method and means for rapid loading and unloading of the work-supporting fixture. In addition, rapid loading and unloading will minimize warming of the opened outer door, heat leak into the insulated chamber, and condensation of moisture on the cold surface. As each of these items contributes to thermal loss in the system, loading time is an important factor to be considered to provide an efficient system and process that consumes the least amount of cryogenic coolant.
Some of the foregoing requirements can be met by the provision of a conventional hook suspended from the roof of the treating chamber from which hook the work-supporting fixture can be suspended, with means being employed for rotating the hook and the suspended fixture. Such an arrangement, however, presents several disadvantages that would be detrimental to the operation of a cryogenic coating removal operation.
A conventional type of hook will substantially delay the loading and unloading of the work-supporting fixture. Each time that the outer door is opened during successive batch operations, the operator must observe the rotational position of the hook and then jog the work rotation drive until the hook has been aligned for engagement with the support fixture. In the possible event that an article falls off the fixture, the conventional hook will continue to transmit the full torque, jamming the rotation system. In such an event the equipment may be damaged by a jam and in certain circumstances the hook may be disengaged and allow the work-supporting fixture to fall. Also, when the door is opened, there is the additional hazard presented that the fixture could topple and cause personal injury to the operator. Further, a conventional hook suspension will allow the fixture to wobble during rotation and thereby cause articles to fall off the fixture.
The above described hazards and problems are obviated or substantially minimized by the novel features of the present invention hereinbelow described.